This application relates to projection apparatus, and particularly to exposure control for an image projection system of the type generally employed in copying machines of the full frame exposure type.
In image projection systems such as copying machines where an image is projected from an illuminated object plane to an image plane the industry has been continually concerned with providing a projected image with consistency of contrast regardless of the background, color, texture or the nature of the image on an original document.
One type of known exposure control device is shown in U.S. Pat. No. 3,379,110 (Blackert et al). In this device one light source of specific wavelength is employed to illuminate an original document, and a second light source of a different wavelength which does not affect the copy material is employed for producing a controlled signal for modulating the intensity of an illumination source.
In U.S. Pat. No. 3,423,153 (Kent), a sensing device detects the light reflected from the face of the orignal document, and a control signal is employed to vary the intensity of the illumination of the light source.
In U.S. Pat. No. 3,426,357 (Paulus), a light transducer detects the intensity of light impinging upon the copy material and various logarithmic amplifier means, multiplier means, summation means and control means are employed for controlling the time of exposure of the original.